The present invention deals with the data transmission in full duplex on a two-wire line, and relates particularly to an echo cancelling device including a phase-roll correction circuit.
A family of modems is for use on connections between data terminal equipments by means of the switched telephone network. The data signals are transmitted from a transmitting modem to a receiving modem over a two-wire line. For long distances, amplifiers (repeaters) are required. Since amplifiers only operate in one direction, the data directions are split up to separate the two-wire line into a four-wire line. The connection between two and four-wire lines, and vice-versa, is made through so called hybrid couplers. Likewise, an hybrid coupler is required to connect the modem output comprising the transmitting circuit and the receiving circuit, to the two-wire line.
An hybrid coupler is a terminating set consisting of two differential transformers with three inputs, an input for each two-wire circuit of the four-wire line, and an input for the two-wire line. Because these transformers cannot be loaded with an impedance matched throughout the overall frequency bandwidth, a portion of the data signal energy goes over to the other direction and returns to the sender through the two-wire line, generating so called echoes.
Two kinds of echoes are received by the modem : the near echo resulting from the transmitted data signal leaking directly through the modem hybrid coupler toward the receiving circuit of the modem, and the far echo resulting from the transmitted data signal passing through the four-wire line and reflecting on a far hybrid coupler.
Therefore, the modems of this type are provided with echo cancellers to cancel from the incoming signal echoes of the outgoing signal. Because near and far echoes have different characteristics, it is classical to have the near echo canceller different from the far echo canceller, the outputs of both echo cancellers being added to provide an estimated value of the actual echo. This resulting estimated value is then subtracted from the incoming signal to provide theoretically the incoming signal free of echo. The error signal between the estimated and actual values of the echo is generally used to adjust the coefficients of the echo cancellers.
In this type of transmission utilizing the switched telephone network the far echo is also affected by a frequency shift. Indeed, the data signal can be frequency switched one or more times, in the equipments of the switched telephone network. As these equipments are not sufficiently interdependent the signal which comes back to the transmitting modem is no longer maintained at the carrier frequency of the latter modem. It results that the phase of the far echo is affected by the so called phase-roll, that is the phase is time variable. The resulting frequency shift is generally low, about 0.1Hz. Such a frequency shift is easily compensated by the adjusting algorithms of the echo canceller, as it is the case for telephone conversation where echo cancellers are continuously in operation since the transmission is always half-duplex (both parties are never talking simultaneously). Unfortunately, in the data transmission over the two-wire lines of the switched telephone network, the adjustment of the echo canceller coefficients cannot be made with sufficient speed and precision during the full duplex transmission.